There has been proposed, for example, a sealing structure of a ceramic package for enhancing the brazing strength of a metallic sealing ring which is brazed at the opening of a cavity of the ceramic package for sealing the cavity, the ceramic package including a first pattern of metalized layer which is formed, through printing, at the outer periphery of the cavity; a second pattern of thick film for receiving a brazing material which is formed, through printing, from a high-melting-point metal on the front surface of the first pattern so as to have a constant width and to extend along an intermediate portion of the first pattern in an inward/outward direction; a third pattern of thin metalized layer which is formed on the aforementioned two patterns so as to have a hat-shaped cross section in an inward/outward direction; the sealing ring (metallic frame) which is brazed onto the third pattern via a brazing material; and a cap (metallic cap) which is welded on the ring (see, for example, Patent Document 1).
Meanwhile, there has been proposed a package for accommodating an electronic component, in which a frame-shaped first metalized layer is formed on almost the entire front surface of an insulating substrate surrounding the opening of a dent (cavity) for accommodating an electronic component therein, the cavity having a quadrangular shape in plan view; and a belt-like second metalized layer having a width smaller than that of the first metalized layer is formed only on each of four side portions of the first metalized layer (i.e., other than corner portions of the front surface) such that the overall thickness of the first and second metalized layers becomes uniform between the aforementioned side portions and corner portions, so as to suppress deformation of a metallic frame brazed on the first and second metalized layers via a brazing material (see, for example, Patent Document 2).
However, when, as described in Patent Document 1, the second pattern of thick film having a constant thickness is formed so as to surround the opening of the cavity and to extend along the entire periphery of the cavity, the thickness of each of the four corner portions is greater than that of each of the four side portions. Therefore, when the sealing ring is brazed via the third pattern of thin metalized layer, the level of the front surface of the ring becomes uneven. Thus, even when a metallic cap is welded on the front surface of the ring, the cavity may fail to be sealed. In addition, since the aforementioned brazing material has a relatively large thickness at the corner portions, the ceramic material may be removed in the vicinity of the front surface at the corner portions, due to thermal stress in association with cooling of the brazing material during brazing of the ring, or due to thermal contraction of the metallic cap joined through welding.
Meanwhile, when, as described in Patent Document 2, the belt-like second metalized layer is formed only on each of the four side portions surrounding the opening of the cavity, a difference in level is generated between the straight side portions and the corner portions. Therefore, when a metallic frame is brazed onto the first and second metalized layers, the metallic frame may be inclined, and thus sealing of the cavity may fail to be achieved by means of a metallic cap welded on the metallic frame.